Modern navigation systems have typically used satellite-based global positioning system (GPS) for position determination. However, the recent proliferation of WiFi access points has made it possible for navigation systems to use these access points for position determination, especially in urban areas where there is usually a large concentration of WiFi access points. WLAN navigation systems can be advantageous over GPS navigation systems because of limitations of GPS signal coverage. For example, while GPS signals may not be readily available inside a shopping mall, wireless signals generated by WiFi access points inside the shopping mall would be more readily detectable by a mobile communication device.
More specifically, for WLAN navigation systems, the locations of the WiFi access points are used as reference points from which well-known trilateration techniques can determine the location of a mobile device (e.g., a WiFi-enabled cell phone, laptop, or tablet computer). The mobile device can use the received signal strength indicators (RSSI) corresponding to a number of visible access points as indications of the distances between the mobile device and each of the detected access points, where a stronger RSSI means that the mobile device is closer to the access point and a weaker RSSI means that the mobile device is further from the access point. The mobile device can also use the round trip time (RTT) of signals transmitted to and from the access points to calculate the distances between the mobile device and the access points. Once these distances are calculated, the location of the mobile device can be estimated using trilateration techniques.
Whether using RSSI or RTT techniques to determine the distances between the mobile device and the visible WiFi access points, the geographic locations (e.g., latitude and longitude) of the access points needs to be known. A number of online databases can be used to determine the locations of large numbers of actively deployed WiFi access points according to their unique basic service set identifier (BSSID) values. For example, companies including Google, Skyhook, Devicescape, and WiGLE have built databases of BSSID values and the geographic locations of the corresponding access points.
However, such WLAN navigation systems are inherently imprecise because different make-and-models of WiFi access points typically have different RSSI and RTT characteristics. For example, an access point manufactured by one company may employ a different RSSI scale (e.g., 1 to 100) than an access point manufactured by another company (e.g., 1 to 127). Similarly, different access point products (even those manufactured by the same company) may have different response times associated with transmitting a beacon signal in response to a probe signal generated by the mobile device. Not knowing the exact response time of a particular access point introduces inaccuracies in the measured RTT. Thus, because of the relatively short broadcast range of WiFi access points (e.g., typically less than 30 meters) in relation to the propagation speed of the WiFi signals, inaccuracies in the calculated RTT resulting from unknown variations in the access points' response times can lead to large errors in the calculated position of the mobile device.
Conventional WLAN navigation and positioning systems typically assume the same estimated RSSI and RTT characteristics for all access points, irrespective their make and model. Thus, because distance calculations using either RSSI or RTT techniques depend upon signal strength scales and processing delays that are specific to individual access points, which in turn typically vary between devices manufactured by different companies and even between different products produced by the same company, such conventional WLAN navigation systems are prone to errors that hinder their accuracy.
Accordingly, there is a need for a system that can consider the varying RSSI scales and/or the varying RTT delay characteristics of a variety of different WiFi access point devices when determining position information of a mobile device using WiFi access points.
Like reference numerals refer to corresponding parts throughout the drawing figures.